(1) Field of the Invention
The present invention relates to a turbine engine component, such as a turbine blade, having modifications which minimize thermal gradients, allows for film cooling, and avoid back flow margin problems as the film holes are fed with a flow of cooling fluid from the main supply cavities.
(2) Prior Art
The overall cooling effectiveness is a measure used to determine the cooling characteristics of a particular design. The non-achievable ideal goal is unity. This implies that the metal temperature is the same as the coolant temperature inside an airfoil. The opposite can also occur where the cooling effectiveness is zero implying that the metal temperature is the same as the gas temperature. In that case, the blade material will certainly melt and burn away. In general, existing cooling technology allows the cooling effectiveness to be between 0.5 and 0.6.
More advanced technology, such as supercooling, should be between 0.6 and 0.7. Microcircuit cooling can be made to produce cooling effectiveness higher than 0.7. FIG. 1 shows a durability map of cooling effectiveness versus the film effectiveness for different lines of convective efficiency. Placed in this map is a point related to the advanced serpentine microcircuits of FIGS. 2A and 2B. FIG. 2A illustrates a suction side cooling circuit 22, while FIG. 2B illustrates a pressure side cooling circuit 20.
Dimensionless Parameters forserpentine microcircuitBeta2.898Tg2581 [F.]Tc1365 [F.]Tm2050 [F.]Tm_bulk1709 [F.]Phi_loc0.437Phi_bulk0.717Tco1640 [F.]Tci1090 [F.]Eta_c_loc0.573Eta_f0.296
Table I provides the dimensionless parameters used to plot the design point in the durability map. It should be noted that the overall cooling effectiveness from the table is 0.717 for a film effectiveness of 0.296 and a convective efficiency (or ability to pick-up heat) of 0.573 (57%).
As illustrated in FIG. 3, the corresponding cooling flow for a turbine blade having this serpentine configuration is 3.5% engine flow. Most traditional high pressure turbine blade designs use about 5% engine flow. As a result, this design leads to significant cooling flow reduction. This in turn has positive effects on cycle thermodynamic efficiency, turbine efficiency, rotor inlet temperature impacts, and specific fuel consumption.
It should be noted from FIG. 3 that the flow passing through the pressure side serpentine microcircuit is 1.165% WAE in comparison with 0.428% WAE in the suction side serpentine microcircuit for this arrangement. This represents a 2.7 fold increase in cooling flow relative to the suction side microcircuit. The reason for this increase stems from the fact that the thermal load to the part is considerably higher for the airfoil pressure side. As a result, the height of the microcircuit channel should be 1.8 fold increase over that of the suction side. That is 0.022 vs. 0.012 inches.
Besides the increased flow requirement on the pressure side, the driving pressure drop potential in terms of source to sink pressures for the pressure side circuit is not as high as that for the suction side circuit. In considering the coolant pressure on the pressure side circuit, at the end of the third leg, the back flow margin, as a measure of internal to external pressure ratio, is low. As a consequence of this back flow issue, the metal temperature increases beyond that required metal temperature close to the third leg of the pressure side circuit.
Since the thermal load to the part is high on the pressure side, particularly toward the aft end of the airfoil, it is desirable to introduce film cooling on that side of the airfoil. However, a low back flow will likely develop when cooling is extracted out of the peripheral circuit. An alternative way around this problem is to introduce film fed from the main cavities. As shown in FIG. 4, there are regions A, B, and C with low heat transfer that define the walls of the cooling channels used in the airfoil. This allows for space between the two legs to increase. This in turn permits EDM hole drilling for film hole cooling the airfoil as shown by the location of the shaped holes D in FIG. 4.
As the peripheral circuits are imbedded in the airfoil wall, regions of high temperature gradients or high heat flux vectors can develop between the legs of the circuit. Thus, it is desirable to reduce these regions of high thermal gradients.